Not Applicable.
Not Applicable.
Not Applicable.
This clip cushion relates to fasteners, and specifically to the aerospace industry where high temperatures are experienced.
The areas around rocket motors and jet engines experience temperature extremes in the 450xc2x0 F. to 600xc2x0 F. range, and above. Assemblies in these areas need to have other parts attached to them. A prime example would be a hydraulic or fuel line attached to or near an engine. Hence, a hollow metal tube needs to be attached to the engine or another object. If a metal strap were used to fasten the metal tube, problems would occur. Fastening one metal part to another causes problems if vibration and movement between parts occur. Also, the metal parts may abrade each other and cause a hole in the tube.
The solution of the 1950""s was to use a non-metallic cushion between the strap and the tube. These parts are usually referred to as clamp cushions or clip cushions.
Aerospace companies approved and wrote specifications around an asbestos textile based product, which was coated with polytetrafluoroethylene dispersion and then dried, formed, sintered, and machined. The entire process took about ten separate steps.
Asbestos in grades AA and AAA and Teflon were serviceable at 600xc2x0 F. The Teflon coating allowed for the tube to slide within the strap during movement. Vibration and abrasion were reduced by the non-metallic cushion effect. However, when the Teflon (i.e. polytetrafluoroethylene) coating wore down, the asbestos fibers caused abrasion due to their high modulus.
However, the use of asbestos came under attack by various government agencies and the public in general.
For whatever reason, the aerospace community eliminated asbestos beginning with product approvals in 1986.
The second solution to this fastening problem was to use a non-asbestos based textile product coated with Teflon as the replacement. The non-asbestos textile was based on Nomex, Kevlar, fiberglass, and polybenzimidazole (i.e. PBI). The process was very similar and also took about ten steps.
However, the replacement product has problem areas. First, Kevlar, Nomex, and PBI are not rated to 600xc2x0 F. They are rated at 355xc2x0 F., 500xc2x0 F., and 500xc2x0 F. respectively as reported in xe2x80x9cMaterial Engineeringxe2x80x94Materials Selector 1989.xe2x80x9d
Second, Kevlar is high modulus and is abrasive.
Third, when a textile-based product is machined, the yarn ends are cut. When this is subsequently bent, the yarn ends have a desire to separate due to stress. This causes a crack between yarn ends which questions product reliability as to whether the crack will propagate over time or not.
Fourth, a ten-step process involving water based dispersions and textile methods can be difficult to control from a composition consistency and dimensional control viewpoints.
Current specifications on the non-asbestos based textile product include:
The invention is an improved product for high temperature clip cushions using an improved composition of matter and an improved process.
The improved process reduces the process steps from about ten to two: mix the powders and ram extrude to size in a Teflon processing extruder. Process inconsistencies are reduced and dimensional values are more tightly controlled through the die forming process.
The improved composition of matter is based on:
Teflon powder (PTFE or chemically modified PTFE*)
Glass bubbles and/or beads (micro version)
High temperature pigments (if needed for coding)
*NXT70 by Dupont and TFM by Dyneon for example.
All of these materials are rated for a minimal weight loss and tensile reduction up to 650xc2x0 F. The Teflon may experience some softening around 635xc2x0 F. Excursions to 700xc2x0 F. are believed possible with minimal problems. Trials on a 70% Teflon and 30% glass bubbles or beads showed a weight loss after 24 hours at 600xc2x0 F. of 0.1% and 0.04% respectively. This compares to a typical 1.00% loss on the standard textile based product.
Abrasion should be improved by the elimination of Kevlar but no tests have been done.
The xe2x80x9ccrackingxe2x80x9d phenomenon between yarn ends of a textile product is eliminated by the use of a homogeneous blend of powders. Bend tests on the materials subjected to 600xc2x0 F. for 24 hours showed full flexibility and no cracking after being bent on themselves (i.e. 180xc2x0 or folded on themselves). The normal test is to bend around a xc2xcxe2x80x3 diameter rod and check for cracks.